The present invention relates generally to an optical recording system, and more.particularly, to a laser driver circuit employed in the optical recording system.
Many optical recording systems use a laser beam to record data in an optical medium such as an optical disk and to retrieve recorded data and other information from the optical medium. In general, the laser power to record data and the laser power to retrieve data are different. The power for writing is usually higher than the power for reading. For example, the laser power for writing data on a magneto-optic medium must be sufficient to raise the temperature at a selected spot above the Curie temperature of the medium. Hence, the laser power needs adjustment to an appropriate level when such a system switches between a writing mode and a reading mode.
A laser in an optical recording system may be the type where the output laser power can be electrically controlled. A semiconductor laser is one example of such a laser and is widely used in various optical recording systems. A laser driver circuit is usually coupled to the semiconductor laser to provide a proper driving current and electrical bias to operate the laser. The magnitude of the driving current can be adjusted to change the laser power.
One embodiment of the invention includes a laser, a photodetector, and a laser driver circuit. The laser produces a laser beam whose power changes in response to a driving signal. The photodetector is coupled to receive a portion of the laser beam to produce a detection signal indicative of the laser power. The laser driver circuit produces the driving signal to the laser. A first signal source is used to produce a first portion of the driving signal and a second signal source Is used to produce a second portion of the driving signal. The laser driver circuit further includes a control circuit to control the second signal source in response to the detection signal to maintain the second portion of said driving signal substantially constant when the first portion of said driving signal changes.
Another embodiment includes a write current source to supply a write current in a write mode and a read circuit to supply a read current in a read mode. A feedforward circuit is connected between the write current source and the read circuit to keep the read current at a substantially constant current level during the write mode.
These and other embodiments can be used in an optical recording system to keep a read laser beam at a substantially constant power level by controlling the read current to the laser, especially when the laser is changed from the write mode to the read mode. Hence, the recovery transient time can be significantly reduced or virtually eliminated.